The use of magnetic field measurements in prior art subterranean surveying techniques for determining the direction of the earth's magnetic field at a particular point is well known. Techniques are also well known for using magnetic field measurements to locate subterranean magnetic structures, such as a nearby cased borehole. These techniques are often used, for example, in well twinning applications in which one well (the twin well) is drilled in close proximity and often substantially parallel to another well (commonly referred to as a target well).
The magnetic techniques used to sense a target well may generally be divided into two main groups; (i) active ranging and (ii) passive ranging. In active ranging, the local subterranean environment is provided with an external magnetic field, for example, via a strong electromagnetic source in the target well. The properties of the external field are assumed to vary in a known manner with distance and direction from the source and thus in some applications may be used to determine the location of the target well. In contrast to active ranging, passive ranging techniques utilize a preexisting magnetic field emanating from magnetized components within the target borehole. In particular, conventional passive ranging techniques generally take advantage of magnetization present in the target well casing string. Such magnetization is typically residual in the casing string because of magnetic particle inspection techniques that are commonly utilized to inspect the threaded ends of individual casing tubulars.
In co-pending, commonly assigned, U.S. patent application Ser. No. 11/301,762 to McElhinney, a technique is disclosed in which a predetermined magnetic pattern is deliberately imparted to a plurality of casing tubulars. These tubulars, thus magnetized, are coupled together and lowered into a target well to form a magnetized section of casing string typically including a plurality of longitudinally spaced pairs of opposing magnetic poles. Passive ranging measurements of the magnetic field may then be advantageously utilized to survey and guide drilling of a twin well relative to the target well. For example, the distance between the twin and target wells may be determined from magnetic field strength measurements made in the twin well. This well twinning technique may be used, for example, in steam assisted gravity drainage (SAGD) applications in which horizontal twin wells are drilled to recover heavy oil from tar sands.
While the above described method of magnetizing wellbore tubulars has been successfully utilized in well twinning applications, there is room for yet further improvement. For example, it has been found that the above described longitudinal magnetization method can result in a somewhat non-uniform magnetic flux density along the length of a casing string at distances of less than about 6-8 meters. If unaccounted, the non-uniform flux density can result in distance errors on the order of about ±1 meter when the distance between the two wells is about 5-6 meters. While such distance errors are typically within specification for most well twinning operations, it would be desirable to improve the accuracy of distance calculations between the target and twin wells.
Moreover, passive ranging surveys are typically acquired at about 10 meter intervals along the length of the twin well. More closely spaced distance measurements may sometimes be advantageous (or even required) to accurately place the twin well. For example, more frequent distance measurements would be advantageous during an approach (also referred to in the art as a landing) or during a period of unusual drift in either the target or twin well. Taking more frequent magnetic surveys is undesirable since each magnetic survey requires a stoppage in drilling (and is therefore costly in time).
Therefore, there exists a need for improved methods for determining the distance between a twin well and a magnetically patterned target well. In particular, there is a need for a method that accounts for fluctuations in magnetic field strength and thereby improves the accuracy of the determined distances. There is also a need for a dynamic distance measurement method (i.e., a method for determining the distance between that does not require a stoppage in drilling).